This invention relates to cast metal turbine wheels with integral airfoil blades. More particularly, the invention relates to making turbine wheels with a radially oriented columnar grain structure in the blades and a substantially equiaxed grain structure in the disc.
Turbine wheels located immediately downstream from the combustion can of a turbine power plant must operate at high temperatures and under mechanical stress generally destructive to cast metal. It is generally known that any defect in an airfoil blade may lead to its failure. One way of reducing the probability of blade failure is to promote the growth of unidirectionally oriented columnar grains therein with grain boundaries substantially parallel to the leading and trailing edges. On the average, such blades are less susceptible to fatigue fracture and have longer service lives. The turbine wheel disc, on the other hand, should have an equiaxed grain structure to more evenly distribute hub forces at typical wheel speeds of 20,000 revolutions per minute or more. Before this invention it was not known how to integrally cast a turbine wheel with columnar grain blades and an equiaxed grain disc. In particular, the relatively large surface area and small volume of the airfoil blades promoted rapid cooling of the metal cast therein with the incumbent formation of equiaxed grains. Although unidirectionally solidified blades might be precast and then attached to an equiaxed disc by powder metallurgy or casting techniques, the finished wheel might be prone to failure under operational stress at the point of blade attachment.